1. Field of the Invention
The present invention relates to a porous metal thin film having a large surface area, a method for manufacturing the same, and a capacitor.
2. Description of the Related Art
In recent years, surfaces of thin films have been roughened in order to improve the functions of the thin films. For example, for the purpose of improving the capacitance of an electrolytic capacitor, a surface of aluminum foil serving as an electrode material is roughened by a chemical conversion treatment so as to increase a surface area.
In a catalyst field as well, platinum black and gold black produced by roughening surfaces of platinum and gold have been used as electrodes and catalysts for various chemical reactions. Furthermore, for example, carbon, ceramic, and metals have been used while surface areas thereof have been increased by surface roughening through conversion to a porous surface for the purpose of improving the functions of automobile exhaust gas treatment catalysts, fuel cell electrode catalysts, TiO2 catalyst electrodes for dye-sensitized solar cells, lithium ion secondary batteries, and the like. Moreover, in the area of field emission displays, electrons are generated by application of a voltage to fine protrusions on an electrode surface, wherein fine protrusions which appear in surface roughening are used.
The surface roughening of a metal film through conversion to a porous surface to increase a surface area, as described above, has been used in wide fields industrially, and further roughening of surfaces has been required. For example, if the surface area of an aluminum surface used for an electrolytic capacitor can be further increased through surface roughening, the performance can be improved.
Therefore, as for a method for facilitating surface roughening, it has been proposed to subjecting aluminum foil for an electrolytic capacitor to surface roughening by using an etching method and an evaporation method in combination to increase the surface area of an electrolytic film and improve the capacitor characteristics. For example, Japanese Unexamined Patent Application Publication No. 63-255910 discloses a method in which a surface of aluminum foil is chemically etched and, thereafter, titanium thin films are deposited sequentially by an evaporation method, so as to facilitate surface roughening. Japanese Unexamined Patent Application Publication No. 2000-299255 discloses a method in which a surface of aluminum foil is roughened through chemical etching and, thereafter, Ti or other metal is deposited. Japanese Unexamined Patent Application Publication No. 10-270291 discloses a method in which a thin Ni film hardly oxidized naturally is evaporated on an aluminum foil surface subjected to an etching treatment, so as to improve the characteristics of a capacitor without impairing an effect of surface roughening of the aluminum foil. Japanese Unexamined Patent Application Publication No. 2006-108159 discloses an electrolytic capacitor including electrode foil in which aggregates of fine grains composed of aluminum provided with an aluminum oxide layer on a surface are adhered to aluminum foil. Japanese Unexamined Patent Application Publication No. 11-317331 discloses a heat-reactive evaporation method in which an evaporation gas is introduced in an atmosphere of inert gas, e.g., nitrogen, containing oxygen while a base material is in the state of being heated.
Incidentally, in contrast to the chemical etching method, the physical evaporation method does not use chemical agents and, therefore, has an advantage that, for example, chemical treatment facilities are not required. Consequently, the physical evaporation method is industrially useful if roughening of a metal thin film surface can be realized by the physical evaporation method alone and, in addition, with a simple process not including heat-formation of film.